Demodulation method

ABSTRACT

The invention relates to a method for incoherent demodulation of orthogonally coded DSSS signals modulated with MSK or similar modulation types. The data signals are retrieved by splitting the received signal into two parts where the carrier is down converted to baseband by a signal (S12, S22) which is not adapted to the phase of the carrier, whereafter the down converted signals pass a correlator (13, 14), quadrature devices (15, 16) and adding devices (17, 18) and finally a discriminator (19) whic always chooses the highest correlation peak as output signals (S30).

The present invention relates to transfer of data by means of directsequence spread spectrum signals (DSSS) where the code is modulated on acarrier by means of MSK (Minimum Shift Keying) or related types ofmodulation, e.g. filtered MSK, and where the receiver does not have torecognize or retrieve the phase of the carrier. This is calledincoherent reception.

MSK is a well known modulation technique. It is for instance describedin the following articles: Carl Erik Sundberg: Continuous PhaseModulation. IEEE Communication Magazine, April 1986; S. Pasupathy:Minimum Shift Keying; A spectrally efficient Modulation. IEEECommunication Magazine, July 1979; F. Amoroso, J. A. Kivett: SimplifiedMSK Signaling Technique. IEEE Transactions on Communications, April1977. The technique is efficient in the sense that the bandwidth whichis required for transferring a certain amount of information per secondis less than what for instance is required in connection with phaseshift keying (PSK). In addition the MSK technique gives a greaterpossibility for operating with a constant envelope in a bandwidthlimited channel than many other modulation techniques. These twocharacteristics make MSK an attractive form of modulation.

DSSS is a well known coding method within military communication wherethe purpose a.o. is to make the receiver less sensitive to disturbingsignals. The method is for instance described in a book by R. C. Dixon:Spread Spectrum Systems, Wiley 1976 and it is based on the principle ofcoding every single data bit with a code which is known by the receiver.The code is usually modulated on the carrier with PSK, but MSK has alsobeen used. The data are usually modulated with differential phase shiftkeying (DPSK).

Orthogonal signalling is a known modulation method which is described ina book by J. G. Proakis: Digital Communications, Mc Graw Hill 1983. Thismodulation method is based on the principle of letting a number of databits (e.g. 1) be represented by a number of different (orthogonal) codes(e.g. 2). The receiver must be able to detect which of the orthogonalcodes (e.g. 2) which was transmitted. When a code has been chosen to bethe correct one, the receiver can determine the data bit which wastransmitted. The method is commonly used for incoherent reception and incases where the bandwidth in a channel is greatly limited.

Incoherent operation is used in receivers which cannot rely on phaseinformation for demodulation of data, because such information is notavailable. This has e.g. been described in the mentioned book by J. G.Proakis. The signal in such a receiver is split in an inphase and aquadrature channel and the signal in each of these is squared, whereuponthe two channels are added. It is impossible to use this method directlyin connection with MSK.

From U.S. Pat. No. 4,583,048 there is known an MSK digital demodulatorfor burst communication. The object of the demodulator described is toundertake a rapid measurement of the phase difference between theincoming carrier and the local reference, so that coherent, conventionalreception of data can take place. It is emphasized in the patent howimportant it is with burst communication systems to regain phasecoherency rapidly, and it is pointed out that this cannot be obtainedwith conventional methods (PPL). The patent does, however, not mentionanywhere how data demodulation is obtained. It appears that only themutual amplitude of the four correlation peaks from each of the twopaths are measured incoherently. These measurements are used to regainthe phase so that the system can be brought to operate as a conventionalcoherent system for demodulation of data.

The purpose of the present invention is to provide a method forincoherent demodulation of a carrier of orthogonally coded DSSS signalswhich are modulated with MSK or related modulation types.

The main features of the invention are defined in the claims. In thepresent system the orthogonal codes are identical with the spreadingcode for the DSSS signals. Extraction of data is obtained by combiningthe above known techniques. The result is that MSK modulated signals canbe received in an incoherent receiver even with very low signal to noiseratio on the radio channel.

Above mentioned and other features and objects of the present inventionwill clearly appear from the following detailed description ofembodiments of the invention taken in conjunction with the drawings,where

FIG. 1 schematically shows a known signal receiver principle,

FIG. 2 schematically shows the principle of the receiver according tothe present invention,

FIG. 3 shows the retrieved signal form in the known receiver,

FIGS. 4 and 5 show an example of retrieved signals in the receiver shownin FIG. 2, for an arbitrary phase.

The associated transmitter uses MSK modulation as described by Amorosoand Kivett in the above mentioned article, in order to modulate acarrier with information. We call the carrier frequency fo and the speedwith which the spreading code is modulated for fc, the chiprate.

An incoming signal S1 contains the code 1110010. In the known receiverin FIG. 1, this signal is mixed in a mixing circuit 1, with a signal S2which can be written as sin (2π(fo-1/4fc)+π/2). The resulting signal S2is indicated in FIG. 3. The mixing signal S2 has the correct phase inthe coherent system. When the samples of the signal are taken at acorrect time, --which requires clock retrieval--, as indicated witharrows, the transmitted information (1110010) may be retrieved directly.A possible signal in an orthogonal channel is not used. The signal S3 isdirected through a correlator 2 to a logic circuit 3 in which there fromthe two input signals S4 and S5 is chosen a signal S6.

The receiver shown in FIG. 2 functions as follows: A signal S10 which isan orthogonally coded DSSS signal modulated with MSK, is first dividedinto two equal parts, S11 and S21 in a splitting circuit 10. The signalparts S11 and S21 are then down converted, in mixing circuits 11 and 12to basebands in two channels having a frequency which differs from thecarrier by 0,25 of fc. The mixer signals S12 and S22 may respectively beexpressed as sin (2π(fo-1/4fc)+φ) and cos (2π(fo-1/4fc)+φ). The phase φis arbitrary so that the baseband signals in each path are notnecessarily similar to the transmitted signals, such as is the case inthe mentioned article. This distinguishes our receiver from all previousreceivers for MSK, as e.g. the one which is shown in the mentionedarticle and which is a coherent receiver for MSK.

In the present system the shapes of the signals S13 and S23 may betotally different from the shape of the signal S3, FIG. 3, as shown inthe FIGS. 4 and 5.

These signal forms are representative for the case where the signal isrespectively 45 and 135 degrees out of phase. Firstly, we can observethat the amplitudes have changed. In addition the peaks have moved tothe left. But it can be shown that if the signals in both parts arecorrelated with the transmitted code for thereafter being squared andadded, the autocorrelation function is constant in position as well asin amplitude, independent of the phase. This is the basis for incoherentdemodulation.

The baseband signals are transferred to a bank of correlators 13 and 14,each being set up with its own code (e.g. 2). After correlation a squarefunction is made in quadrature devices 15 and 16 and the signals arefinally added in adding devices 17 and 18. In a logic circuit ordiscriminator device 19 the amplitudes of the signals S14 and S24 arecompared on the outputs (e.g. 2), and the code which gives the highestcorrelation peak is declared as the one that was sent, as represented bythe signal S30.

Before the data transmission can start the receiver will have to besynchronized with the transmitter, i.e. the time of arrival for thecodes must be determined. This synchronizing system is not shown.

The above detailed description of embodiments of this invention must betaken as examples only and should not be considered as limitations onthe scope of protection.

I claim:
 1. Method for demodulating data represented by orthogonallycoded Direct Sequence Spread Spectrum Signals (DSSS) which in atransmitter is modulated on a carrier with MSK (Minimum Shift Keying)(or related continuous phase modulation types) and which in a receiveris down converted to baseband with a frequency which is shifted with 1/4chiprate relatively to the center frequency of the received signal bytwo signals which are 90 degrees out of phase with each other, in twoorthogonal channels with individual identical correlators (or banks ofcorrelators), squared in quadrature devices and thereafter added inadding devices, characterized in this that information about thetransmitted code and thereby also about the transmitted data areextracted in a discriminator device by choosing the code which gives thehighest correlation peak out of the adding devices in spite of the factthat the phase difference between the signal which is used for downconversion to baseband and the center frequency of the transmittedsignal remains unknown and even slowly varying (incoherentdemodulation), and in spite of the fact that the modulation method usedto modulate the carrier is MSK or a related modulation type.
 2. Methodaccording to claim 1, characterized in this that each of the downconverted signals are correlated in at least two paths and are squaredin individual devices.
 3. Method according to claim 2, characterized inthis that individual parts of the correlated, squared signals are addedin individual adding devices for producing comparable added signals.